Spin is a kind of angular momentum that fundamental particles have. It doesn’t have a classical analogue.
It is an intrinsic property of elementary particles on one hand, and a quantized observable which behaves like the angular momentum from classical mechanics on the other. Similarly to how mass is the energy associated to some particles just by their existence, spin is the angular momentum associated to some particles just by their existence. And just as there are massless particles like photons, there are spin-0 particles like the Higgs boson. In this sense, it is “something real and measurable, just like mass and charge”.
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